Which phenomenon produces upward force as the liquid moves toward the outlet weirs?

• A. Frictional Drag
• B. Buoyancy
• C. Surface Loading
• D. Hydrostatic Pressure

Answer: (C)

The key idea is how the free surface of a moving liquid interacts with a constriction like an outlet weir. As water in a channel moves toward the weir, the flow geometry changes the way the surface carries and transmits load to the liquid just upstream of the weir. This vertical transfer of load through the free surface is called surface loading. It effectively creates a vertical force component on the liquid as the flow accelerates and the surface area exposed to the constriction changes.

Frictional drag acts along surfaces and resists motion, not as a vertical force pushing the liquid upward toward the weir. Buoyancy is the upward force on submerged objects due to the surrounding fluid, which isn’t the mechanism here since we’re dealing with the moving surface of the liquid itself. Hydrostatic pressure is the pressure due to the weight of the liquid at depth and acts mostly downward on surfaces; it doesn’t explain the upward force arising from the changing surface flow toward the weir.